Vegetation Community Characteristics along the Northwest Fork of the Loxahatchee River and Development of a Salinity-Vegetation Model Technical Publication WS-19 By John G. Zahina Water Supply Department February 2004 South Florida Water Management District Loxatchee River Vegetation Studies and Model Acknowledgements ACKNOWLEDGEMENTS This document was produced within the Ecological Evaluation Group of the Water Supply Planning and Development Division. Thanks to Joel VanArman and Dave Swift for encouraging field studies and development of the SAVELOX model. Acknowledgement goes to Patti Sime and Dan Haunert of the Northern District Restoration Division of the South Florida Water Management District (SFWMD or District) for providing the means for review and publication of this document. Thanks to Steve Hill for statistical analysis and Gordon Hu for providing output from the Hydrodynamic Salinity Model for the Northwest Fork of the Loxahatchee River. Winnie Park-Said and Cathy McCarthy provided much-appreciated field study assistance. The document has been greatly improved by constructive and thorough review comments provided by District staff including John Mulliken, Dawn Rose, Cathy McCarthy, Mike Duever, Lou Toth, Ken Konyha, Gordon Hu and Dan Crean. Scientific peer review was provided by Dr. Merry Alber (University of Georgia), Dr. Donald M. Kent (Community Watershed Fund, Florida) and Dr. Christopher Craft (Indiana University). i Acknowledgements Loxahatchee River Vegetation Studies and Model ii Loxatchee River Vegetation Studies and Model Executive Summary EXECUTIVE SUMMARY The purpose of this report is to document the methods and results from vegetation studies conducted by South Florida Water Management District (SFWMD or District) staff along the Northwest Fork of the Loxahatchee River (Northwest Fork) and the development of a salinity-vegetation (SAVELOX) model derived from these studies. A section containing basic information concerning movement of salt water within the Northwest Fork and the dominant species found within the river floodplain is included. This background information is essential to understand the system and rationale supporting development of the SAVELOX model. SFWMD staff conducted vegetation surveys at floodplain sites from 2000-2002. Two different methods were used in these surveys; a semiquantitative survey was conducted first at more than twenty locations in order to gain an understanding of the distribution of species across this system and to discern the locations of fresh water, transitional and salt-tolerant floodplain communities. More intensive, quantitative surveys focused on studying vegetation changes along this gradient with particular emphasis on documenting physical differences of dominant species and community structure. Because long-term salinity monitoring data is not available for all vegetation sites surveyed along the Northwest Fork, a hydrodynamic salinity model was used to generate a “historic” 30-year salinity time series (1971–2001). This model was calibrated and verified using existing salinity-freshwater flow relationships derived from field studies. The long-term salinity time series at each vegetation site was used to calculate the Ds/Db ratio, which is a means to integrate salinity exposure duration, magnitude and recovery time between salinity events into a single numerical factor. This ratio was related to quantitative vegetation survey data to examine relationships between salinity conditions and stress to freshwater vegetation. A model was developed from relationships between field data and long-term salinity conditions. The SAVELOX model uses an empirical approach to extrapolate vegetation parameter response given a set of long-term salinity conditions. A comparison of model output with field data indicates that it provides a reasonable estimate of the status of the vegetation community at a site. The predicted values calculated by the SAVELOX model are useful to examine salinity-vegetation relationships and to compare potential changes in the distribution of freshwater habitat along the Northwest Fork associated with different flow scenarios. This model can be a useful tool for evaluating hydrological modeling alternatives relative to restoration efforts. iii Executive Summary Loxahatchee River Vegetation Studies and Model iv Loxatchee River Vegetation Studies and Model Table of Contents TABLE OF CONTENTS ACKNOWLEDGEMENTS ..............................................................................................I EXECUTIVE SUMMARY ............................................................................................III INTRODUCTION............................................................................................................. 1 BACKGROUND INFORMATION ................................................................................ 3 DESCRIPTION OF THE LOXAHATCHEE RIVER .................................................................... 3 CHARACTERISTICS OF SALINITY MOVEMENT IN THE NORTHWEST FORK.......................... 5 LOXAHATCHEE RIVER HYDRODYNAMIC MODEL.............................................................. 8 Model Description........................................................................................................ 8 Modeling Assumptions ................................................................................................. 9 FLOODPLAIN SWAMP FLORISTICS ................................................................................... 10 Ecological Characteristics of the Floodplain Swamp Forest ...................................... 9 Bald Cypress .............................................................................................................. 11 Other Freshwater Swamp Species.............................................................................. 12 Red Mangrove ............................................................................................................ 13 SALINITY-VEGETATION RELATIONSHIPS ALONG THE NORTHWEST FORK....................... 15 FLOODPLAIN VEGETATION STUDIES ............................................................................... 17 General Land Office Survey of Township & Range Plats.......................................... 17 United States Geological Survey Topographic Map of the Jupiter Area................... 17 Alexander & Crook .................................................................................................... 18 Floodplain Transect Study along the Northwest Fork- Various Authors .................. 19 U.S. Department of the Interior Environmental Impact Statement............................ 21 METHODS ...................................................................................................................... 22 DEVELOPMENT OF A LONG TERM SALINITY TIME SERIES .............................................. 23 FIELD VEGETATION SURVEYS ........................................................................................ 25 Semiquantitative Vegetation Survey........................................................................... 25 Quantitative Vegetation Survey.................................................................................. 26 STATISTICAL ANALYSIS OF FIELD DATA ........................................................................ 29 SALINITY-VEGETATION RELATIONSHIPS AND DEVELOPMENT OF A MODEL ................... 30 RESULTS ........................................................................................................................ 35 DEVELOPMENT OF A LONG TERM SALINITY TIME SERIES .............................................. 36 VEGETATION SURVEY RESULTS ..................................................................................... 36 Semiquantitative Vegetation Survey........................................................................... 36 Quantitative Vegetation Survey.................................................................................. 43 LOXAHATCHEE SALINITY-VEGETATION (SAVELOX) MODEL VERIFICATION AND RESULTS......................................................................................................................... 51 DISCUSSION .................................................................................................................. 55 RECENT VEGETATION CHANGES ALONG THE NORTHWEST FORK ................................... 55 APPLICATION AND INTERPRETATION OF SAVELOX OUTPUT ........................................ 57 v Table of Contents Loxahatchee River Vegetation Studies and Model CONCLUSIONS ............................................................................................................. 61 RECOMMENDATIONS................................................................................................ 63 REFERENCES................................................................................................................ 65 APPENDIX A: CSCE/EWRI & ICECM Conference Preceedings…………..…....A-1 APPENDIX B: General Land Office 1855 Survey Notes…………………………...B-1 APPENDIX C: Field Studies Vegetation Data……………………………..…..……C-1 APPENDIX D: SAVELOX Model Verification……………………………………..D-1 vi Loxatchee River Vegetation Studies and Model Introduction INTRODUCTION Floodplain vegetation along downstream segments of the Northwest Fork of the Loxahatchee River (Northwest Fork) (Figure 1) has changed over the past century from freshwater swamp dominated by bald cypress (Taxodium distichum) to salt-tolerant red mangrove (Rhizophora